Mounting structure of semiconductor device and mounting method thereof

ABSTRACT

An insulating sheet which connects a semiconductor chip and a wiring substrate is provided between the semiconductor chip and the wiring substrate. The insulating sheet has windows therethrough at positions corresponding to those of connection pads of the wiring substrate and has leads, one end of each of the leads being fixed on the sheet and the other end of each of the leads protruding from the opposite surface of the sheet through a window. Each of solder balls of the semiconductor chip is connected to the fixed one end of one of the leads, and each of the connection pads is connected to the other end of each of the leads to electrically connect the semiconductor chip and the wiring substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 09/788,596, nowU.S. Pat. No. 6,803,647, filed on Feb. 21, 2001, the entire contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mounting structure of a semiconductordevice which can be easily detached from a wiring substrate after beingmounted on the wiring substrate and a mounting method thereof.

2. Description of the Related Art

Conventionally, a method is known where a semiconductor chip is mountedon a wiring substrate through solder balls densely provided on onesurface of the semiconductor chip. In such a conventional mountingstructure of a semiconductor device, resin referred to as underfill isfilled into the gap between the semiconductor chip and the wiringsubstrate. For example, Japanese Patent Application Laid-open No. Hei10-284635 discloses a semiconductor device in which filling resin isembedded in a gap between a semiconductor chip and a substrate so as tocover solder balls.

With reference to FIG. 7, in such a conventional semiconductor device, asemiconductor chip 21 and a wiring substrate 25 are connected to eachother by welded solder balls 26 of the semiconductor chip 21. Further,resin 29 is injected into the gap between the semiconductor chip 21 andthe wiring substrate 25 so as to cover the solder balls 26. The resin 29is injected for the purpose of alleviating the thermal stress caused bythe difference in the coefficient of thermal expansion between thesemiconductor chip 21 and the wiring substrate 25. The semiconductorchip 21 and the wiring substrate 25 repeat expansion and contraction byheat generated by operations (on/off operation) of the device. However,the coefficient of thermal expansion of the semiconductor chip 21 isabout 3.5 ppm whereas the coefficient of thermal expansion of the wiringsubstrate 25 is about 16 ppm in case of a printed board and about 8 ppmin case of an alumina substrate. Due to this difference in thecoefficient of thermal expansion between the semiconductor chip 21 andthe wiring substrate 25, the solder balls 26 are alternately subject tocompressive stress and tensile stress. As a result, the solder balls 26are broken at an early stage due to thermal fatigue, which causeselectric disconnection, resulting in a signal transmission stop or apower supply stop.

Therefore, by filling the resin 29 into the gap between thesemiconductor chip 21 and the wiring substrate 25 so as to cover thesolder balls 26. The resin 29 alleviates stress on the solder balls 26.This suppresses the deterioration of the solder balls 26, and thereliability of the connection between the semiconductor chip 21 and thewiring substrate 25 is improved. It is to be noted that epoxy-basedresin is mainly used as the resin 29.

However, in the conventional semiconductor device described above, sincethe semiconductor chip 21 and the wiring substrate 25 are mechanicallyfirmly bonded to each other with the resin 29, once the semiconductorchip 21 is attached to the wiring substrate 25, the semiconductor chip21 can not be easily detached from the wiring substrate 25. Therefore,there has been a problem in that the semiconductor chip 21 can not beeasily replaced and the maintainability is lowered.

An object of the present invention is to provide a mounting structure ofa semiconductor device in which a semiconductor chip can be detachedfrom a wiring substrate and high reliability is realized.

According to the present invention, a mounting structure of asemiconductor device with excellent connection reliability can alleviatethe stress on solder balls caused by the difference in the coefficientof thermal expansion between a semiconductor chip and a wiring substratewhen the semiconductor chip is mounted on the wiring substrate.

Further, according to the present invention, amounting structure of asemiconductor device with excellent maintainability can be provided inwhich a semiconductor chip can be easily detached from a wiringsubstrate after the semiconductor chip has been mounted on the wiringsubstrate.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, in a mounting structureof a semiconductor device, an insulating sheet having a plurality ofleads is attached between a semiconductor chip and a wiring substrate,the plurality of leads electrically connecting a plurality of solderballs and a plurality of corresponding connection pads, respectively.

The insulating sheet has holes therethrough at positions correspondingto those of the plurality of connection pads.

One end of each of the plurality of leads is fixed on a first surface ofthe insulating sheet, while the other end is inserted in one of theholes.

Each of the plurality of solder balls is electrically connected to thefixed one end of a corresponding one of the plurality of leads, whileeach of the plurality of connection pads is electrically connected tothe other end of a corresponding one of the plurality of leadsprotruding from one of the holes.

According to another aspect of the present invention, a mounting methodof a semiconductor device includes providing an insulating sheet havingholes therethrough at positions corresponding to those of a plurality ofconnection pads and having a plurality of leads, one end of each of theplurality of leads being fixed on a first surface of the insulatingsheet and the other end of each of the plurality of leads protrudingfrom a second surface of the insulating sheet through one of the holes,electrically connecting the other end of each of the plurality of leadsof the insulating sheet to a corresponding one of the plurality ofconnection pads of a wiring substrate, and electrically connecting eachof a plurality of solder balls of a semiconductor chip to the fixed oneend of a corresponding one of the plurality of leads.

According to still another aspect of the present invention, aninsulating sheet has a plurality of holes therethrough and a pluralityof leads, one end thereof being fixed on a first surface of theinsulating sheet and the other end thereof being shaped to be afloat inthe holes.

According to yet another aspect of the present invention, a method ofmanufacturing an insulating sheet comprises the steps of providing ametal film on one surface of the insulating sheet, masking and etchingthe metal film to form a plurality of leads, cutting out predeterminedplaces of the insulating sheet to provide a plurality of holes throughthe insulating sheet, and making one end of each of the plurality ofleads fall into a corresponding one of the plurality of holes.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the invention willbecome more fully apparent from the following detailed description takenin conjunction with accompanying drawings:

FIG. 1 illustrates the structure of a first embodiment according to thepresent invention;

FIG. 2 is a perspective view illustrating the structure of the firstembodiment;

FIGS. 3( a) to 3(e) illustrate a method of manufacturing an insulatingsheet;

FIGS. 4( a) to 4(d) illustrate a mounting method of the mountingstructure according to the first embodiment;

FIG. 5 illustrates another structure of an insulating sheet;

FIG. 6 illustrates the structure of a second embodiment according to thepresent invention; and

FIG. 7 illustrates a conventional structure of a semiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of a mounting structure of a semiconductor device isnow described-in-detail with reference to the drawings. With referenceto FIGS. 1 and 2, the first embodiment of the present invention includesa semiconductor chip 1, an insulating sheet 2, and a wiring substrate 5.

A plurality of pads 3 for electric connection to the external arearranged to be lattice-like on one surface of the semiconductor chip 1.Solder balls 6 are attached to the plurality of pads 3, respectively. AStannum/Plumbum(Sn/Pb) alloy, an Stannum/Bismuth/Argentum (Sn/Bi/Ag)alloy, a Stannum/Silver (Sn/Ag) alloy or the like is used for thecomposition of the solder balls 6.

The wiring substrate 5 has therein internal wirings 7 formed of aconductive material such as Copper (Cu). Further, the wiring substrate 5has on its surface where the semiconductor chip 1 is mounted a pluralityof connection pads 8 corresponding to the pads 3 of the semiconductorchip 1, respectively. The respective plurality of connection pads 8 areconnected to the internal wirings 7 to be electrically connected to apower source layer or other components.

The insulating sheet 2 is formed of an insulating material such aspolyimide resin, Teflon resin, epoxy resin and alumina at the thicknessof about 0.1 mm–0.5 mm. The coefficient of thermal expansion of theinsulating sheet should be between the coefficient of thermal expansionof the semiconductor and the coefficient of thermal expansion of thewiring substrate. The insulating sheet 2 also has windows 10therethrough formed by cutting out places corresponding to theconnection pads 8 on the wiring substrate 5. Furthermore, the insulatingsheet 2 has a plurality of leads 4, one end (hereinafter referred as afirst end)of each leads being fixed on the insulating sheet 2 and theother end (hereinafter referred as a second end) of each leads beinginserted into the windows 10 to be in a floated state. First ends 11 ofthe leads 4 are fixed on the insulating sheet 2 at positionscorresponding to the solder balls 6 of the semiconductor chip 1,respectively. Moreover, the leads 4 are curved or bent so that unfixedsecond ends 12 protrude through the windows 10 from a opposite surfaceof the insulating sheet 2. The unfixed second ends 12 of the leads 4 arepositioned to places corresponding to the places of the connection pads8 on the wiring substrate 5, respectively.

A method of manufacturing the insulating sheet 2 is described. Withreference to FIGS. 3( a) to 3(e), at first, a metal film 14 is formed onan insulating sheet 13 by plating or the like (FIG. 3( a)). Then, usinga mask of a desired pattern (in this embodiment, the pattern of leads),the metal film 19 is etched to form leads 15 (FIG. 3( b)). Some parts ofthe insulating sheet 13 are cut out to provide windows 16 (FIG. 3( c)).Then, one end of each of the leads 15 is made to fall into one of thewindows 16 as second end (FIG. 3( d)).

Another method of manufacturing the insulating sheet 2 is as follows.First, the windows 16 are provided in the insulating sheet 13 (FIG. 3(e)). Then, one end of each of the plurality of leads 15 prepared inadvance is fixed to the insulating sheet 13 as first end (FIG. 3( c)).After that, the other end of each of the plurality of leads 15 as secondend is made to fall into one of the windows 16 (FIG. 3( d)).

A mounting method of the mounting structure of the semiconductor deviceaccording to the present invention is described in detail with referenceto the drawings.

With reference to FIGS. 4( a) to 4(d), the insulating sheet 2 is mountedon the wiring substrate 5 with the surface of the insulating sheet 2where second ends 12 are protruding therefrom being facing the wiringsubstrate 5. Here, the second ends 12 of the leads 4 are aligned andconnected to the corresponding connection pads 8 on the wiring substrate5, respectively (FIG. 4( a)). The second ends 12 may be connected to theconnection pads 8 by pressing or soldering. This electrically connectsthe leads 4 to the wiring substrate 5 below the windows 10 through thesecond ends 12 and the connection pads 8. Then, the semiconductor chip 1is mounted on the surface of the insulating sheet 2 which is opposite tothe surface connected to the wiring substrate 5, that is, on the surfaceof the insulating sheet 2 having first ends 11 fixed thereto. The solderballs 6 are aligned and connected by welding to the corresponding thefirst ends 11, respectively (FIG. 4( b)). This electrically connects thesemiconductor chip 1 to the wiring substrate 5 through the leads 4provided on the insulating sheet 2, which makes it possible toinput/output an electric signal and supply power between thesemiconductor chip 1 and the wiring substrate 5.

According to the first embodiment of the present invention, since thefirst ends 11 of the leads 4 are formed on the insulating sheet 2, evenwhen the solder balls 6 are welded and mounted onto the first ends 11 ofthe insulating sheet 2, sufficient connection strength can bemaintained. Furthermore, the connection pads 8 are connected to theunfixed second ends 12 of the leads 4. Therefore, even when the wiringsubstrate 5 is deformed and stress is caused, the resilience of theleads 4 can absorb and alleviate the stress.

Therefore, even when thermal stress is caused by the difference in thecoefficient of thermal expansion between the semiconductor chip 1 andthe wiring substrate 5, the resilience of the leads 4 absorbs the stressto alleviate the stress on the solder balls 6. Accordingly,disconnection at the solder balls 6 is prevented to improve thereliability of the connection.

As a result, the necessity to encapsulate the solder balls 6 of thesemiconductor chip 1 with resin or the like is eliminated, and thus,even after the semiconductor chip 1 is mounted on the wiring substrate5, the semiconductor chip 1 can be easily detached from the wiringsubstrate 5 by fusing the solder balls 6. Therefore, the maintainabilityof the semiconductor chip 1 is improved.

It is to be noted that the leads 4 provided for the insulating sheet 2may be any resilient conductive material, including Cu and Au.

It is preferable that the first ends 11 of the leads 4, to which thesolder balls 6 are to be welded, are appropriately processed into acircular shape, a polygonal shape or the like to fit the shape of thesolder balls 6.

Furthermore, With reference to FIG. 5, though FIG. 2 illustrates thestructure where one window 10 is provided with regard to the pluralityof connection pads 8, the structure may be that a window is providedwith regard to each of the connection pads 8.

Next, a second embodiment according to the present invention isdescribed in detail with reference to the drawings.

With reference to FIG. 6, similarly to the structure of the firstembodiment, connection pads 8 on a wiring substrate 5 are connected tosecond ends 12 of leads 4, and first ends 11 of the leads 4 areconnected to solder balls 6 of a semiconductor chip 1 by welding thesolder balls 6, respectively.

However, the second embodiment differs from the first embodiment in thatresin 9 for encapsulation is filled into the gap between the wiringsubstrate 5 and an insulating sheet 2 and into windows 10 in theinsulating sheet 2 when the wiring substrate 5 is connected to theinsulating sheet 2.

This can reinforce the connection between the second ends 12 of theleads 4 and the connection pads 8. Thus, the connection between theinsulating sheet 2 and the wiring substrate 5 can be ensured.Furthermore, the resilience of the resin 9 is added to the resilience ofthe leads 4, which further alleviates stress.

In this case, after the second ends 12 are connected to the connectionpads 8 as illustrated in FIG. 4( a), the resin 9 is filled into the gapbetween the insulating sheet 2 and the wiring substrate 5 and into thewindows 10 (FIG. 4( c)). After that, the first ends 11 are connected tothe solder balls 6, and the semiconductor chip 1 is mounted on theinsulating sheet 2 (FIG. 4( d)).

According to the second embodiment, similarly to the case of the firstembodiment, since the leads 4 absorb and alleviate the thermal stresscaused between the semiconductor chip 1 and the wiring substrate 5, thestress on the solder balls 6 is alleviated to improve the reliability ofthe connection.

Furthermore, even after the semiconductor chip 1 is mounted on thewiring substrate 5, the semiconductor chip 1 can be easily detached fromthe wiring substrate 5 by fusing the solder balls 6, which improves themaintainability.

Moreover, by filling the resin 9 for encapsulation into the gap betweenthe wiring substrate 5 and the insulating sheet 2 and into the windows10 in the insulating sheet 2, the connection between the second ends 12and the connection pads 8 can be ensured without impairing theresilience of the leads 4.

As is clear from the above description, according to the presentinvention, by utilizing the resilience of the leads of the insulatingsheet, the thermal stress caused by the difference in the coefficient ofthermal expansion between the semiconductor chip and the wiringsubstrate can be absorbed. As a result, the stress on the solder ballsis alleviated to improve the reliability of the connection between thesemiconductor chip and the wiring substrate.

Furthermore, since the necessity to cover the solder balls with resin iseliminated, even after the semiconductor chip is mounted on the wiringsubstrate, the semiconductor chip can be easily detached from the wiringsubstrate by fusing the solder balls. Therefore, the semiconductor chipcan be easily replaced, which improves the maintainability of thesemiconductor device.

Moreover, by filling the resin for encapsulation into the gap betweenthe wiring substrate and the insulating sheet and into the windows inthe insulating sheet, the connection between the leads and theconnection pads can be ensured without impairing the resilience of theleads.

Furthermore, the structure where the insulating sheet is sandwichedbetween the semiconductor chip and the wiring substrate makes itpossible to mount the semiconductor chip on various wiring substrates ofvarious shapes, which enhances the degree of freedom of thesemiconductor device.

While the present invention and its advantages have been described inconjunction with preferred embodiments in the above detaileddescriptions, the present invention is not limited thereto, and variouschanges, substitutions and alternations can be made therein withoutdeparting from spirits and scope of the inventions as defined by theappended claims.

1. A mounting method of a semiconductor device for mounting asemiconductor chip provided with a plurality of solder balls on a wiringsubstrate provided with a plurality of connection pads, comprising:providing an insulating sheet having holes corresponding to saidconnection pads and having a plurality of leads, one end of each of saidleads being fixed on a first surface of said insulating sheet and theother end of each of said leads protruding from a second surface of saidinsulating sheet through one of said holes; electrically connecting saidother end of each of said leads of said insulating sheet to acorresponding one of said connection pads; and electrically connectingeach of said solder balls to said fixed one end of a corresponding oneof said leads.
 2. The mounting method of a semiconductor device asclaimed in claim 1, further comprising: filling resin into the gapbetween said insulating sheet and said wiring substrate after connectingsaid other end of each of said leads to a corresponding one of saidconnection pads.